1. Field of the Invention
The present invention relates to a scanner for reading a color image which is suitably applied to a digital color image telegraphy apparatus, a color image telegraphy printer and so on.
2. Description of the Prior Art
A proposed scanner for reading a color image as part of a digital color image telegraphy apparatus, a color image telegraphy printer or the like is shown in FIG. 1, which is a cross-sectional view thereof, (see official gazette of Japanese laid-open patent application No. 58-212255).
As shown in FIG. 1, in this conventional scanner, a reading or scanning section 2 is located closely opposed to an original document 1. By "document" is meant any medium on which a color image can be formed, such as a photograph, printed image, fine art, or the like. On the original document 1, a color image is formed by some suitable method such as a printing-process or the like. The scanning section 2 is provided with a CCD (charge-coupled-device) line sensor 3 whose photo or image-detecting portion 3a is opposed to the original document 1. The photo-detecting portion 3a has a length (taken in the direction into the plane of the drawing as viewed in FIG. 1) corresponding to the width of the original document 1. A multi-lens-array 4 which is comprised of a plurality of rod-shaped lenses, each lens being aligned in a straight line configuration, is located between the photo-detecting portion 3a of the CCD line sensor 3 and the original document 1 such that the light of the image corresponding to one line of the original document 1 in its width direction can become incident on the photo-detecting portion 3a of the CCD line sensor 3 through the multi-lens-array 4.
A fluorescent light lamp or source 5 is provided proximate to the side wall of the multi-lens-array 4 for emitting a blue light. The lamp 5 has a length (again, taken in the direction perpendicular to the plane of FIG. 1) corresponding to the width of the original document 1. When the fluorescent light source 5 is lit, the original document 1 is irradiated with the blue light. A light emitting diode unit 6 is located or mounted on the scanning section 2 proximate to the side wall of the multi-lens-array 4 which is opposite to the fluorescent light source 5. The length of the light emitting diode unit 6 corresponds to the width of the original document 1.
Referring to FIG. 2, the light emitting diode unit 6 comprises a base plate 6a and a series of red and green light emitting diodes 6R and 6G which are alternately aligned on the base plate 6a in a straight line, wherein when the red light emitting diodes 6R are lit, they irradiate the original document 1 with a red light while when the green light emitting diodes 6G are lit, they irradiate the original document 1 with a green light.
Referring back to FIG. 1, the original document 1 close to the scanning portion 2 is transported by a plurality of rollers 7. The original document 1 is transported stepwise by the rollers 7 and at every line of the original document 1 it is irradiated with the red light by the red light emitting diodes 6R, the green light by the green light emitting diodes 6G and the blue light by the fluorescent light source 5 repeatedly in a time-division-manner. Then, a reflected, multicolored light image of the original document 1 is detected and is read by the photo-detecting portion 3a of the CCD line sensor 3. The colored, light image formed on the original document 1 is analyzed into the red, green and blue primary colors at every line so that the primary color signals of the color image forming one picture are read by transporting the original document 1 over its entire length.
Since the light emitting diodes are used as the light source as described above, the prior art scanner can be simplified in arrangement and the power consumed for the reading operation can be reduced.
The wavelength region of the light from the light emitting diodes are very narrow or severe so that neutral color light between (i.e. except) red, green and blue cannot be read precisely by the conventional scanner having the above-mentioned arrangement.
Specifically, the light emitting characteristics of the light emitting diodes 6R and 6G and that of the fluorescent light source 5 are generally demonstrated as shown in a graph forming FIG. 3, in which the red light emitting diodes 6R emit a light whose peak wavelength .lambda.p is equal to about 660 nanometers, the green light emitting diodes 6G emit a light whose peak wavelength .lambda.p is equal to about 555 nanometers and the blue fluorescent light source 5 emits a light of which the peak wavelength .lambda.p is equal to about 450 nanometers. Also, the half widths of the red and green lights from the light emitting diodes 6R and 6G are as narrow as about 30 nanometers, so that the respective light emitting wavelength regions do not have any overlap at all. For this reason, the prior art scanner as mentioned above cannot identify by its output signal a subtle difference between color tones of yellow group colors having a wavelength of about 600 nanometers which are between those of, for example, red and green.
Further, since the green light emitting diodes 6G are low in light emitting efficiency, the green light must be irradiated on the original document 1 for a duration of time longer than that of the other color lights and the scanning operation therefore needs plenty of time.